The present study summarizes the results of our research on Provitamin D photochemistry in various media directed on the development of reliable method for an in situ monitoring of the Vitamin-D-synthetic capacity (and ‘antirachitic’ UV dosimetry) of sunlight and artificial UV radiation. Such measurements are especially important given the essential role of vitamin D in maintaining health, as well as the observed pandemic of vitamin D deficiency among the world's population. The original UV biodosimeter is based on the same photoreaction in vitro by which vitamin D is synthesized in human skin from initial Provitamin D via photo- and thermo-induced monomolecular isomerizations . The targets for UV photons in the biodosimeter are the 7-Dehydrocholesterol (Provitamin D3) molecules embedded in specially designed UV transparent and stable matrix. The dosimeter response to UV radiation is photoinduced conversion of Provitamin into Previtamin D (immediate precursor of vitamin D), and antirachitic UV biodose is determined by the amount of accumulated previtamin D. Effect of a reaction medium on the Provitamin D photochemistry was studied in polymer and hydrogel films , in micelles and in liquid crystalline matrices . Specificity of interaction of provitamin D molecules with different microenvironment affecting its photoconversions is considered.